The present invention relates to a laminate-type photoconductor for electrophotography (hereinafter simply referred to as a "photoconductor") installed on an electrophotographic apparatus which uses coherent or interfering light as a light source. The present invention also relates to a photoconductor which can obtain excellent images free from interference fringes and image defects.
Many conventional photoconductors used in the electrophotographic apparatuses including copying machines, printers and facsimiles which employ Carlson method have used inorganic photoconductive materials, such as selenium, selenium alloys, zinc oxide, and cadmium sulfide. Recently, organic photoconductors which use organic photoconductive materials have been developed vigorously in view of the environmental considerations, ease of film formation and light weight. Among the organic photoconductors, so-called function-separation-type organic photoconductors which include a separate charge generation layer and charge transport layer have been mainly used, because in case the respective layers are formed of the most suitable materials, there are many merits, such as the greatly improved sensitivity and the adjustable spectroscopic sensitivity considering the desired wavelength of the exposure light.
The function-separation-type organic photoconductor generally includes a charge generation layer on a conductive substrate and a charge transport layer on the charge generation layer. The charge generation layer and the charge transport layer constitute a photoconductive film. The photoconductor is manufactured by forming the charge generation layer on the conductive substrate by coating and drying the coating liquid containing a charge generation agent and a binder dispersed and dissolved in a solvent, and by successively forming the charge transport layer on the charge generation layer by coating and drying the coating liquid containing a charge transport agent and a binder dispersed and dissolved in a solvent.
The basic functions required to the photoconductor are obtained by laminating the charge generation layer and charge transport layer directly on the substrate. However, the charge generation layer is generally 0.5 .mu.m or less in thickness for quickly injecting charge carries generated by absorbed light into the substrate and the charge transport layer. Due to the thin charge generation layer, film defects, such as pin holes and non-uniformity, are formed if there are flaws, stains and deposits on the substrate surface. The film defects further cause image defects, such as black spots and uneven printing density. Also, since the charge injection between the substrate and the charge generation layer is not so well prevented, the charge retention rate of the photoconductor is lowered by the holes injected from the substrate. Lowering of the charge retention rate further causes fogging on a white sheet of paper.
It is known that a resin intermediate layer is formed between the substrate and the photoconductive film to prevent image defects by the non-uniformity of the charge generation layer and hole injection from the substrate.
The resins used for the intermediate layer include solvent-soluble polyamide, polyvinyl alcohol, polyvinyl butyral and casein. The intermediate layer that uses one of these resins exhibits its basic functions even when it is extremely thin, e.g., 0.1 .mu.m or thinner. However, the intermediate layer should have the thickness of 0.5 .mu.m or more to cover the defects and stains on the conductive substrate surface or to prevent the non-uniformity of the charge generation layer. Sometimes, the intermediate layer should have the thickness of 1 .mu.m or more depending on the roughness and contamination of the substrate surface.
The thick intermediate layer as described above affects adversely to the carrier injection from the charge generation layer, and causes residual potential rise after repeated use and image defects, such as lowering of printing density. The thick intermediate layer also causes wide variation of the electrical properties of the photoconductor depending on the environment in which the photoconductor is used. Especially, when the photoconductor is used in a hot and very humid environment, the electrical resistance is changed greatly by the dissociation of water absorbed in the thick intermediate layer, and fogging is formed on a solid white image.
Various materials have been proposed for providing such thick intermediate layer with low electrical resistance that changes little with environmental change. Japanese Patent Publications (KOKAI) No. H02-193152, No. H03-288157 and No. H04-31870 disclose solvent-soluble polyamide resins whose chemical structures have been identified. Japanese Patent Publications (KOKAI) No. H02-59458, No. H03-81778 and No. H02-281262 disclose additives for polyamide resin for preventing the electrical resistance from changing by the environmental change. Japanese Patent Publications (KOKAI) No. H03-145652, No. H03-81778 and No. H02-281262 disclose the mixtures of polyamide resin and other resins for adjusting the electrical resistance to suppress the environmental influence on the electrical resistance.
As to the other materials for the intermediate layer, Japanese Patent Publication (KOKAI) No. H02-238459 discloses the employment of cellulose derivatives, Japanese Patent Publications (KOKAI) No. H02-115858 and No. H02-280170 disclose the employment of polyether urethane. Japanese Patent Publication (KOKAI) No. H02-105349 uses polyvinyl pyrrolidone, and Japanese Patent Publication (KOKAI) No. H02-79859 uses polyglycol ether.
When the photoconductor including one of these intermediate layers is used for the laser beam printer, it is necessary to prevent the image defect of interference fringes due to the combination of the refractive index and thickness of the photoconductive film and the wavelength of the light source. In order to prevent the interference fringes, it has been proposed to add an inorganic pigment filler to the intermediate layer. The proposed inorganic pigment fillers for the intermediate layers include small grained aluminum oxide (Japanese Patent Publication (KOKAI) No. H03-24558) and a large amount of rutile-type titanium oxide mixed in acrylmelamine (Japanese Patent Publication (KOKAI) No. H02-67565). An intermediate layer of 2 to 10 .mu.m in thickness that uses anatase-type titanium oxide with the purity of 99% or more has been proposed to improve the dispersing ability of the filler and the electrical properties of the intermediate layer (Japanese Patent Publication (KOKAI) No. H04-172361). This patent publication discloses that anatase-type titanium oxide is superior to rutile-type titanium oxide for improving the dispersing ability of the filler and for lowering the electrical resistance of the intermediate layer.
Japanese Patent Publication (KOKAI) No. S60-144754 discloses an intermediate layer that includes binder resin and conductive powder consisting of coarse powder component of 0.2 to 0.6 .mu.m in average particle diameter and fine powder component of 0.1 .mu.m or less in average particle diameter. However, this combination of the coarse and fine powder components is not so effective to prevent the interference fringes due to the coherent light.
Although various intermediate layers between the conductive substrate and the photoconductive film have been proposed, it is still desirous to develop a laminate-type photoconductor, adaptable to the electrophotographic apparatus which uses coherent or interfering light, such as a laser beam, as a light source, which prevents interference fringes and image defects and which can provide excellent images.
In view of the foregoing, it is an object of the invention to provide a photoconductor for preventing interference fringes and image defects and providing excellent images.